https://en.wikipedia.org/wiki/Nuclear_marine_propulsion

The attraction is that high enrichment permits a smaller reactor and less frequent refuelling. US submarines go over 96% enrichment.

Bunker fuel is in the news. Cheap oil for marine diesel engines is both polluting in the traditional sulfur etc sense and in the modern CO2 global warming sense. There are occasional attempts at wind powered sailing ships, or at least sail assisted ships, to reduce the burning of bunker fuel. Nuclear power for shipping is currently out of fashion, but is a legitimate topic for research.